1. Field of the Invention
The present invention relates to a small gap measuring apparatus and more particularly to an apparatus well suited for optically measuring, with a high degree of accuracy, the gap between the opposing mask and wafer surfaces or the proximity gap in a proximity exposure system used in the manufacture of semiconductor integrated circuits.
2. Description of the Prior Art
In a proximity exposure system of the type in which a mask and a wafer are opposed with a small gap therebetween and the pattern on the mask is printed on the wafer, the gap between the mask and the wafer must accurately be preset to the desired value.
For this purpose, various apparatus have been proposed in the past in which the alignment is achieved by detecting the positions vertical to the surfaces of a mask and wafer. For example, a proximity exposure system is known which uses a gap detecting apparatus for detecting the position of a wafer surface relative to a mask surface by means of capacitance changes.
However, more accurate alignment has been required every year for these exposure systems and it has been impossible to obtain satisfactory accuracy with the conventional methods.